The present invention is directed to organic light emitting devices (OLEDs) comprised of emissive layers that contain phosphorescent dopant porphyrin molecules having reduced symmetry for producing a saturated red emission with greater brightness.
Organic light emitting devices (OLEDs) are comprised of several organic layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device, C. W. Tang et al., Appl. Pys. Lett 51, 913 (1987). Certain OLEDs have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, P. E. Burrows and M. E. Thompson, Laser Focus World, February 1995). Since many of the thin organic films used in such devices are transparent in the visible spectral region, they allow for the realization of a completely new type of display pixel in which red (R), green (G), and blue (B) emitting OLEDs are placed in a vertically stacked geometry to provide a simple fabrication process, a small R-G-B pixel size, and a large fill factor, U.S. Pat. No. 5,707,745. This patent disclosed a stacked OLED (SOLED) for which both intensity and color could be independently varied and controlled with external power supplies in a color tunable display device. Each layer in the integrated SOLED was independently addressable and emitted its own characteristic color. This colored emission could be transmitted through the adjacently stacked, transparent, independently addressable, organic layer or layers, the transparent contacts and the glass substrate, thus allowing the device to emit any color that could be produced by varying the relative output of the color-emitting layers. U.S. Pat. No. 5,707,745, thus, illustrates a principle for achieving integrated, full color pixels that provide high image resolution, which is made possible by the compact pixel size. Furthermore, relatively low cost fabrication techniques, as compared with prior art methods, may be utilized for making such devices.
A transparent OLED (TOLED), V. Bulovic, G. Gu, P. E. Burrows, M. E. Thompson, and S. R. Forrest, Nature 380, 29 (1996), which represents a further significant step toward realizing high resolution, independently addressable stacked R-G-B pixels, was reported in U.S. Pat. No. 5,703,436, in which the TOLED had greater than 71% transparency when turned off and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode and a Mg-Ag-ITO electrode layer for electron-injection. A device was disclosed in which the ITO side of the Mg-Ag-ITO electrode layer was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED.
Such devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, OLEDs are comprised of at least two thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material""s ability to transport holes, a xe2x80x9chole transporting layerxe2x80x9d (HTL), and the material of the other layer is specifically selected according to its ability to transport electrons, an xe2x80x9celectron transporting layerxe2x80x9d (ETL). With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the hole transporting layer, while the cathode injects electrons into the electron transporting layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring, under certain conditions, preferentially via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
The materials that function as the electron transporting layer or as the hole transporting layer of the OLED are frequently the same materials that are incorporated into the OLED to produce the electroluminescent emission. Such devices in which the electron transporting layer or the hole transporting layer functions as the emissive layer are referred to as having a xe2x80x9csingle heterostructurexe2x80x9d (SH). Alternatively, the electroluminescent material may be present in a separate emissive layer between the hole transporting layer and the electron transporting layer in what is referred to as a xe2x80x9cdouble heterostructurexe2x80x9d (DH).
In addition to emissive materials that are present as the predominant component in the charge carrier layer, that is, either in the hole transporting layer or in the electron transporting layer, and that function both as the charge carrier material as well as the emissive material, the emissive material may be present in relatively low concentrations as a dopant in the charge carrier layer. Whenever a dopant is present, the predominant material in the charge carrier layer may be referred to as a host compound or as a receiving compound. Materials that are present as host and dopant are selected so as to have a high level of energy transfer from the host to the dopant material. In addition, these materials need to be capable of producing acceptable electrical properties for the OLED. Furthermore, such host and dopant materials are preferably capable of being incorporated into the OLED using starting materials that can be readily incorporated into the OLED by using convenient fabrication techniques, in particular, by using vacuum-deposition techniques.
It is desirable for OLEDs to be fabricated using materials that provide electroluminescent emission in a relatively narrow band centered near selected spectral regions, which correspond to one of the three primary colors, red, green and blue so that they may be used as a colored layer in an OLED or SOLED. It is also desirable that such compounds be capable of being readily deposited as a thin layer using vacuum deposition techniques so that they may be readily incorporated into an OLED that is prepared entirely from vacuum-deposited organic materials.
U.S. Ser. No. 08/774,087, now U.S. Pat. No. 6,048,630, which is incorporated herein in its entirety by reference, is directed to OLEDs containing emitting compounds that produce a saturated red emission. The emission layer is comprised of an emitting compound having a chemical structure represented by Formula I: 
wherein
X is C or N;
R8, R9 and R10 are each independently selected from the group consisting of hydrogen, alkyl, substituted alkyl, aryl and substituted aryl; wherein R9 and R10 may be combined together to form a fused ring;
M1 is a divalent, trivalent or tetravalent metal; and
a, b and c are each 0 or 1;
wherein, when X is C, then a is 1; when X is N, then a is 0;
when c is 1, then b is 0; and when b is 1, c is 0.
The examples disclosed in co-pending U.S. Ser. No. 08/774,087, now U.S. Pat. No. 6,048,630, which is incorporated herein in its entirety by reference, included an emissive compound of formula I wherein X=C; R8=phenyl; R9xe2x95x90R10xe2x95x90H; c=0; and b=1. This compound has the chemical name 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP). OLEDs comprised of the TPP-containing emissive layer produce an emission spectrum comprised of two narrow bands that are centered at about 650 and about 713 nm, as shown in FIG. 1. The emission from this device involves fluorescence from the TPP dopant. One of the problems with the TPP-doped device is that the narrow band at 713 nm, which comprises about 40% of the emission, is not within a range that is useful for display applications. A second problem is that TPP-doped OLEDs are very unstable, such that the shelf life of such devices is typically very short. It would be desirable if these two aspects of TPP-doped devices could be improved. The present invention is directed to addressing these problems of prior art devices.
Another aspect of the present invention relates to the fact that, based on spin statistical arguments, it is generally understood that the majority of the excitons that are produced in an OLED are in a non-emissive triplet electronic state. Formation of such triplet states can result in a substantial loss of the excitation energy in the OLED via radiationless transitions to the ground state. It would be desirable if the total OLED quantum efficiency could be enhanced by utilizing this energy transfer pathway through the exciton triplet states, for example, by having the exciton triplet state energy transferred to an emissive material. Though it was known that the energy from an excited triplet state could be efficiently transferred under certain circumstances to the triplet state of a molecule that phosphoresces, prior to the disclosures contained in co-pending application having Ser. No. 08/980,986, now U.S. Pat. No. 6,303,238, which is incorporated herein in its entirety by reference, it had been thought that the phosphorescent decay rate would not be expected to be rapid enough to be adequate for use in a display device. U.S. Pat. No. 6,303,238 discloses that, in fact, practical OLEDs can be fabricated in which the emissive layer includes a phosphorescent compound. As a specific representative embodiment of such phosphorescent compounds, a platinum octaethylporphine (PtOEP) compound was disclosed having the chemical structure with the formula: 
Whenever such compounds are doped, for example, into the Alq3 layer of an OLED comprised of layers of ITO/TPD/Alq3/Mgxe2x80x94Ag in sequence, the OLED was found to produce useful external quantum efficiencies with a very narrow emission having a half-width of about 30 mn centered at about 645 nm. Such a narrow band is of particular interest for use in OLEDs, since this band produces what is perceived to be a saturated red emission.
Though PtOEP may itself yet prove to have the most desirable combination of properties for use in OLEDs, such a compound has the disadvantage of producing the saturated red emission near the edge of the eye sensitivity curve, for which the standardized CIE photopic response function of the human eye is centered at about 550 nm. In particular, at the blue and red ends of the spectrum, there is a steep reduction in the eye sensitivity as a function of wavelength.
It would be desirable if phosphorescent compounds could be found which produce what is perceived to be a saturated red emission with a high external quantum efficiency, but with narrow peaks at somewhat shorter wavelengths for which there is a substantially higher eye sensitivity. For example, if the emission bandwidth could be kept substantially constant and the emission peak shifted about 20 nm toward shorter wavelengths, for an OLED producing the same number of photons, for example, an OLED with the same current and quantum yield, the perceived brightness of the device could be increased by a factor of about two. That is, though the number of photons coming from the two devices would be the same, the standard observer would perceive a factor of two increase in brightness for the device having the peak at the shorter wavelength. The emission would, however, be in a region that would still be perceived as saturated red and, thus, still be useful in an OLED.
The present invention is directed to materials and methods that may be used for fabricating OLEDs that address this objective.
The present invention is directed to OLEDs, and a method of fabricating OLEDs, in which emission from the device is obtained via a phosphorescent decay process wherein the phosphorescent decay rate is rapid enough to meet the requirements of a display device.
More specifically, the present invention is directed to OLEDs comprised of a material that is capable of receiving the energy from an exciton singlet or triplet state and emitting that energy as phosphorescent radiation.
One of the benefits of the present invention is that the phosphorescent decay process utilizes exciton triplet state energy that is typically wasted in an OLED via a radiationless energy transfer and relaxation process.
The present invention is further directed to materials and methods for fabricating OLEDs in which a phosphorescent dopant compound produces a highly saturated red emission in a spectral region for which the photopic response function for the human eye is significantly increased as compared with previously disclosed phosphorescent compounds.
In particular, the phosphorescent compounds of the present invention produce a highly saturated red emission with improved luminous efficacy as compared with OLEDs comprised of PtOEP, a compound that produces a narrow emission band that peaks at about 645 nm when the PtOEP is doped in an electron transporting layer comprised of tris-(8-hydroxyquinoline)-aluminum (Alq3).
More specifically, the present invention is directed to a method of selecting phosphorescent dopant compounds for use in an OLED, wherein the phosphorescent compound is selected to be a platinum-porphine compound having reduced symmetry as compared with the 4-fold symmetry of compounds such as PtOEP, so as to obtain compounds having an emission peak shifted toward the peak of the eye sensitivity curve, while still remaining in a spectral region that is perceived as saturated red.
Further objectives and advantages of the present invention will be apparent to those skilled in the art from the detailed description of the disclosed invention.